We just redid my '77 with very a very similar setup.
70cc Treats party with the "custom cut sport/touring" head. 15mm bing (with MLM intake and air filter).
techno circuit (not proma circuit) pipe - so close.
Our timing is at around 14.5-15ish. 14-45 gears (stock). We get 38 mph out of it on the flat, max.. We have an 85 jet in it... it does 4-stroke a little at the top end- I have an 84 on order.
My concern is heat - what temps are you getting? When I drive it up a big (and we have *steep* hills here) - we are close to 400. Normal operating like 340. I changed the front gear to 16, but it didn't really help with the heat, and really made it sluggish on take-off.
Treats said it should be in the 320-340 operating normally... but we shouldn't be pushing the 400.